1. Field of the Invention
This invention relates to treatments for cleaning boreholes in a subterranean formation and particularly to sweeps for removing cuttings from boreholes. More particularly, this invention relates to methods for evaluating the efficiency or effectiveness of sweeps for removing cuttings from boreholes.
2. Description of Relevant Art
Rotary drilling methods employing drilling apparatus having a drill bit and drill stem have long been used to drill boreholes or wellbores in subterranean formations. Drilling fluids or muds are commonly circulated in the well during such drilling to serve a number of functions, including cooling and lubricating the drilling apparatus, counterbalancing the subterranean formation pressure encountered, and removing drill cuttings from the formation out of the wellbore. In removing drill cuttings from the well, drilling fluids suspend the cuttings and carry them to the surface for removal from the well.
Drilling deviated, horizontal and extended-reach wells has become increasingly common in the oil and gas industry. In drilling such wells, gravity causes deposits of drill cuttings, and especially fines or smaller sized cuttings, to build up along the lower or bottom side of the wellbore. Such deposits are commonly called “cuttings beds.” As used herein, the term “deviated” with respect to wells shall be understood to include any well at sufficient angle or deviation off of vertical that cuttings beds tend to form during the drilling operation. “Deviated” wells shall be understood to include without limitation “angled,” “high-angled,” “oval,” “eccentric,” “directional,” and “horizontal” wells, as those terms are commonly used in the oil and gas industry. The terms “well,” “wellbore” and “borehole” are synonymous unless indicated otherwise.
Cleaning (i.e., removing drill cuttings from) a deviated well, particularly drilled at a high angle, can be difficult. Limited pump rate, eccentricity of the drill pipe, sharp build rates, high bottom hole temperature and oval shaped wellbores can all contribute to inadequate hole cleaning. In turn, inadequate hole cleaning can lead to cuttings bed build-up in the wellbore, because commonly used drilling fluids can sometimes fail to remove cuttings from cuttings beds while circulating through the wellbore.
Even in vertical wells, the drilling fluid is not always able to remove drill cuttings efficiently and consequent accumulation can occur. Buildup of cuttings beds can lead to undesirable friction and possibly to sticking of the drill string. Such buildup is especially a problem in extended-reach drilling and in wells using invert emulsion type drilling fluids.
Well treatments or circulation of fluids, called sweeps or sometimes pills, specially formulated to remove these cuttings beds (and other cuttings that would normally not be brought out of the wellbore by the base drilling fluid system) are periodically used to prevent buildup to the degree that the cuttings or fines interfere with the drilling apparatus or otherwise with the drilling operation. These sweeps typically have Theological or density properties significantly different from those of the base drilling fluid system being used, and these sweeps or pills typically are formulated in small volumes (e.g., less than 150 bbl).
Sweeps are commonly applied in vertical as well as in deviated and extended reach drilling applications. The following basic types of sweeps are used in the field: low viscosity; high viscosity; high density; and tandem sweeps comprised of any two of these three preceding types of sweeps. Depending on the nature of a specific drilling operation, sweeps are used to augment cleaning in intervals ranging from a few hundred feet to over 35,000 feet in length (or depth) and at angles ranging from 0° to about 90° from vertical. Commonly, the drilling operation must be stopped while such treatment fluids are swept through the wellbore to remove the fines. However, U.S. Pat. No. 6,290,001 for “Method and Composition for Sweep of Cuttings Beds in a Deviated Borehole” of West et al., assigned to Halliburton Energy Services, Inc., discloses a sweep material and method that can be used without stopping the drilling operation.
The drilling literature contains many references to the use of sweeps and their successes and failures in specific applications. Determining whether a particular type of sweep will bring out large volumes of cuttings from wells has been hard to predict and thus the choice of a particular sweep for a particular operation may be difficult. Often a trial and error procedure is used to decide which type or types of sweeps should be used and how often the sweeps should be used.
Visual estimates of quantities of drill cuttings removed from a well with drilling fluid are commonly made to ascertain the need for a sweep and then to ascertain the effectiveness of the sweep. Sometimes cuttings are collected below the separation shakers and quantities of cuttings wet with drilling fluid are measured on a volume or weight basis.
These methods of evaluating the effectiveness of sweeps are known to have problems or deficiencies. The common method of using an individual's perception of the quantities of drill cuttings coming across the shakers is subject to inaccuracies due to the subjective nature of the method. Two or more individuals seeing the same phenomenon may estimate the quantities of cuttings quite differently. The method involving collection of cuttings in boxes and measuring their volume as a function of time (e.g., the number of seconds or minutes to fill up a box of a given volume) can be quite labor intensive. The volumes must be converted to an estimate of drill cuttings collected on a weight basis by running laboratory tests to determine the amount of liquid drilling fluid adhering to a given weight of cuttings. Often when invert emulsion drilling fluids are used, the drilling mud contains a base oil, weighting material, formation samples, water, and a salt dissolved into the water to obtain desired drilling fluid properties. The laboratory work and the various calculations needed to determine the dry mass of the formation cuttings inherently contain errors that reduce the accuracy of the final estimate of dry cuttings. Further, any fine cuttings that pass through the separation shaker screens will not be collected in the cuttings boxes nor will they be visible to an individual watching drill cuttings pass over the separation shakers.
Modeling of drilling fluid circulating hydraulics to incorporate the effects of sweeps can also be done. Such models are usually sophisticated and many produce results within a reasonable range of error. However, known models do not rely on actual measurement of drilling fluid density or drill cuttings concentration in the annulus.
There continues to be a need for improved methods for determining the effectiveness and efficiency of sweeps in removing residual cuttings and cuttings beds from a wellbore during a drilling operation.